Biological materials, and in particular biomass fuels, are commonly burnt in processes for generation of heat and energy. One of the most important biomass fuels is wood. However, different biomass fuels generate different amounts of heat and different amounts and types of residuals after burning. Great deviations exist also for different types and qualities of wood. This makes it difficult to control a burning or combustion process effectively.
Thus, it is often of great importance to be able to estimate the heating value of a biological material. For example in bio-energy systems, including burning systems, it is of great importance to estimate the heating value of the material fed to the bio-energy system, in order to control the burning process more precisely, and improve its efficiency. The heating value typically varies between different types of biological materials, but also within each type. For example, the same type of biological material may have different moisture content, different ash properties, etc. For example in wood, this may depend on a variety of factors including the type of tree or shrub, the part of the tree or shrub (bark, wood, leaves), etc.
Many suggestions have been proposed during the years to provide estimates of heating value of different materials. For example, U.S. Pat. No. 7,690,268 discloses a method for determining heating value of a flowing material. However, this method can only be used on a single, predetermined material, for which the calorific values are known beforehand. Thus, this method cannot be used when many different materials are used simultaneously. Similarly, the method disclosed in U.S. Pat. No. 3,934,139 is also related to estimation of heating value for one specific material, and also requires determination of the density of the material. The method disclosed in EP 0 718 553 determines the moisture content of a material, and assumes that this is correlated to the heating value. Even though this assumption may be correct for some materials, it is not generally valid, which makes the method difficult to use for systems handling a variety of biological materials. Further, common problems with such known methods are that that the apparatuses are large and expensive, that the methods are relatively tedious and cumbersome to perform, and/or that the results are imprecise and unreliable.
In WO 13/004593 by the same applicant, a much improved method was presented, where an estimate of the heating value was obtainable directly from transmission measurements obtained when irradiating the sample at two different energy levels of e.g. X-ray, and correlating this with reference data obtained through e.g. adiabatic bomb calorimeter measurement. However, sometimes there is a relatively large content of non-combustible elements in the biological material, such as calcium, potassium and silica, e.g. in the form of oxides and/or carbonates, magnesium, phosphor, and heavy metals, such as cadmium, zinc and copper. The content of such non-combustible elements may also vary significantly over time, and from sample to sample. In such cases, it may be difficult to use this previously known method for determination of heating values efficiently and accurately.
There is therefore a need for an improved method for a fast and reliable estimation of the heating value of a biological material, and in particular a method which can be used also when handling a variety of biological materials, and with variable content of non-combustible elements.